Code
P1185
OLDSMOBILE
P — Powertrain
Engine Oil Temperature Circuit
AI status
Completed
Completed
100%
Causes
- Faulty engine oil temperature sensor (thermistor)
- Open or shorted wiring between sensor and ECM
- Corroded, loose or damaged connector terminals
- Poor ground or reference voltage to the sensor circuit
- ECM input circuit fault or internal ECU failure
- Aftermarket modifications or oil cooler sensor relocation error
Symptoms
- Illuminated check engine/malfunction indicator lamp (MIL)
- Incorrect oil temperature reading on dash (if equipped)
- Hard starting, rough idle or rich/lean adjustment issues in some cases
- Reduced engine performance or fallback fueling strategy
- Possible failure to reach normal closed‑loop operation
What to check
- Retrieve freeze frame and live data for oil temp with a scan tool
- Perform visual inspection of sensor, harness and connector for damage/corrosion
- Backprobe sensor signal and reference pins; note voltage/resistance values
- Measure sensor resistance cold and warm and compare to expected behavior (NTC decreases resistance as temp rises)
- Check continuity and for shorts to battery or ground between sensor and ECM
- Monitor for intermittent changes while flexing harness or running engine to warm
Signal parameters
- Sensor type: typically NTC thermistor (resistance decreases as temperature increases)
- Typical signal range to ECM: 0–5 V depending on sensor and circuit (varies by model)
- Open-circuit symptom: signal voltage near 5 V or very high resistance
- Short-to-ground symptom: signal voltage near 0 V or very low resistance
- Expected resistance example (generic): several kiloohms at cold, lower at operating temperature — consult factory data for exact values
Diagnostic algorithm
- Use a scan tool to confirm P1185 is current and view live oil temperature value; note freeze frame data.
- Visually inspect sensor and wiring for damage, oil contamination, loose connectors or corrosion; repair as needed.
- With ignition ON (engine off), backprobe the sensor connector. Verify the reference voltage (often 5 V or pull-up) and signal behavior while grounding/unplugging the sensor to see expected changes.
- Measure sensor resistance at ambient temperature and after warming (remove sensor or disconnect harness as needed). Confirm resistance changes with temperature; compare to service specifications.
- Check continuity between sensor signal pin and the ECM input pin; check for short to battery or ground.
- If wiring and sensor test good, swap with a known good EOT sensor or replace sensor and retest.
- If fault persists with good sensor and wiring, inspect ECM connector pins for damage. Consider ECM bench test or replacement only after wiring/sensor are proven good.
- Clear codes, perform a road/drive cycle or warm-up and verify closure of the fault and correct live oil temp readings.
Likely causes
- Damaged sensor harness (chafing, broken wire, pinched)
- Connector corrosion or bent pins at sensor or ECM
- Sensor failed from age/contamination
- Short to battery or ground on the sensor signal wire
- Intermittent connection due to heat or vibration
Fault status
Status
Stored when the engine control module detects an abnormal oil temperature circuit signal (open, short, out-of-range or erratic). The ECM may substitute a default oil temperature value, which can affect fuel control and engine protection strategies. Further investigation of the sensor, wiring, connectors and ECM input is required.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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